This invention relates generally to a method for monitoring the concentration of certain constituents of interest in the blood of a patient, particularly the blood gases oxygen and carbon dioxide, as well as the concentration of hydrogen ions (pH).
It is important in the management of critically ill human patients that it be known how well the patient's life sustaining functions are performing. It has long been a goal to develop a technique for continuously monitoring the major functions of a patient. Some form of monitoring is considered to be essential in many circumstances, such as for a patient who is receiving intensive care or for a patient undergoing serious surgery. One major life sustaining function is pulmonary ventilation; that is, the function performed primarily by the lungs in exchanging carbon dioxide in a patient's blood for fresh oxygen from the air. Another major function is blood circulation of the patient which carries the blood's oxygen and carbon dioxide between the lungs and various portions of the body. A third major function is the patient's metabolism; that is, the ability of body cells and tissues to function. Monitoring of these major life sustaining functions permits corrective action to be taken in time to maintain life should a function be detected to be failing. The corrective action is usually taken almost without regard to the underlying disease which is causing the function to fail.
Blood circulation is satisfactorily observed by present techniques and available equipment that monitor blood pressure and pulse rate of a patient. However, satisfactory techniques are not now available for continuously observing the ventilation and metabolism functions. The attempts to satisfactorily monitor these functions date a long way back in the history of medicine. An early technique for monitoring pulmonary ventilation involved monitoring the air expelled from the patient's mouth during breathing for its carbon dioxide and oxygen content to determine if the oxygen/carbon dioxide exchange is being performed satisfactorily by the lungs and other organs. This technique is inadequate since in many critically ill patients large alveolar-arterial gradients develop due to atelectasis, arteriovenous shunting, and deterioration of the mechanical functions of the lungs. It is also awkward since it interferes with the patient's normal breathing.
Presently utilized techniques for monitoring the adequacy of pulmonary ventilation only provide for intermittent sampling and measuring of the partial pressure (concentration) of oxygen and carbon dioxide and the concentration of hydrogen ions (pH) in the blood. The level of oxygen measures the probability that a patient's tissues are receiving adequate nutrition. If there is not a sufficient oxygen level in the blood, respiratory therapy with blood transfusions or fluid replacement may be made to temporarily improve tissue oxygenation while the underlying cause for the problem is being diagnosed and treated.
The partial pressure or concentration of carbon dioxide in the blood and blood acidity as measured by concentration of hydrogen ions (pH) provides information as to the extent of the metabolic stress under which the patient's tissues are laboring. Unfavorable metabolic conditions as detected by monitoring carbon dioxide and pH can be attacked directly by respiratory therapy and intravenous fluids while the underlying disease causing the imbalance is being diagnosed and treated.
Such derangement of major life sustaining functions in a patient are corrected, at least temporarily, independently of the underlying disease which caused the condition. Once corrected, the tissues and the vascular system are better able to resume normal operation that help maintain life. Early detection of changes in blood gas and pH values is essential in order to prevent a cardiovascular catastrophe but no accurate and fast device is commercially available for continuously detecting such blood chemistry changes. Alterations in blood gas tension and pH often precede such function changes as can be detected by other existing techniques such as monitoring arterial and venous pressure, use of an electrocardiogram or electroencephalogram or by observing the outward clinical appearance of the patient.
The most straight forward approach to these blood chemistry measurements is to periodically withdraw an arterial blood sample from the patient and then test it to determine the various concentrations. This technique suffers the disadvantage of not continuously providing the desired information, requiring a significant amount of blood to be withdrawn from the critically ill patient over a period of time and further requiring manual implementation. Furthermore, the data obtained represent the blood constituent concentrations only at an instant in time and does not reflect trends. Inaccuracies of such techniques, even with elaborate testing devices, is discussed by Adams et al in Anaestheia, Vol. 22, No. 4, Oct., 1967, pp. 575-597. Serious complications, thrombosis and embolism, may develop in patients during and following arterial puncture or catheterization as reported by Formanek et al in Circulation, XLI, pp, 833-839 (1970).
There has been a great deal of effort in the past to develop an accurate, real time technique for monitoring blood gas tension and pH. A substantial amount of work has been performed by others in the direction of developing extracorporeal instruments to which the patient's circulatory system is coupled in order to provide more or less continuous analysis of blood gas tension and pH data. For example, such work has been reported by Walton et al in Biomedical Sciences Instrumentation, Vol. 7, pp. 155-158, and by Clark et al in Computers and Biomedical Research, 4, pp. 262- 274 (1971).
Another approach for continuous measurement of these parameters which has received a great deal of attention in the past decade is the temporary implanting in an artery a device for detecting the concentration of one or more constituents of interest in the patient's blood. One class of such devices utilizes a gas permeable membrane at the end of a catheter that is extended into an artery with the blood gases permeating therethrough. These gases extracted from the catheter assembly by a pump without withdrawing blood. These gases are analyzed by a mass spectrometer, or some other device, substantially continuously. Such inter-arterial techniques are described in numerous papers in the technical literature, such as by Brantigan et al in the Journal of Applied Physiology, Vol. 28, No. 3, March 1970, pp. 375-377, and by Dardik et al in Surgery, Gynecology and Obstetrics, December 1970, pp. 1157-1160. Another intra-arterial technique includes the direct insertion of a miniaturized pH electrode at the tip of a catheter for the measurement of blood pH. One such device is described by Band et al in Journal of Applied Physiology, 22(4), 1967, pp. 854-857.
Yet another blood gas tension and pH measuring approach receiving some attention during the past decade and which overcomes the implantation disadvantage is to monitor gases passing through the skin. The content of these gases is proportional to the concentration of blood gases and the pH of blood within the patient. This approach is based upon the knowledge that blood vessels are distributed to an area just below the skin in a quantity that supplied blood in excess of the needs of the skin. The measurement of carbon dioxide and oxygen, for example, escaping through the skin from these blood vessels is an indication of the concentration and partial pressure of such constituents in the arterial blood of the patient.
One specific application of the skin gas measurement approach has been to position an inlet device of a mass spectrometer against the skin of a patient and pumping desired escaping gases therefrom for analysis. Another suggested technique, as described in U.S. Pat. No. 3,659,586 - Johns et al (1972) and by Johns et al in Biomedical Sciences Instrumentation, Vol. 5, 1969, pp. 119-121, teaches the use of a carbon dioxide sensor for positioning on the patient's skin over a "window" are wherein the top most dry skin layer (stratum corneum) of the epidermis has been removed, according to the method reported by Pinkus in Journal of Investigative Dermatology, 16, pp. 383-386 (1951) and its effects as reviewed by Tregear in Physical Functions of Skin, p. 21 (1966). Carbon dioxide permeates a membrane of the electrode into its interior, causing conductivity changes of a liquid therein which is measured as an indication of the carbon dioxide concentration of the blood. A similar type of skin electrode suggested to be used without stripping the skin is described in West German Pat. No. 2,145,400, and the German publication Biomedizizische Technik, 1973, No. G, pp. 216-221.
Despite all of the effort, as summarized above, to develop a satisfactory technique for continuous monitoring of blood gas tension and pH, no continuous real time monitoring technique is presently in commercial use. There is a very strong need for a simple, accurate and safe monitoring apparatus. However, present efforts are primarily limited to improve the accuracy of existing intra-arterial and skin electrode devices.